A metallocene complex having one cyclopentadiene ligand is a compound which is called a half metallocene complex and utilized as a catalytic component for polymerizing various olefins, and it is known that the property thereof (a catalytic activity on a polymerization reaction) greatly varies depending on the kind of its central metal. Inter alia, it is known that a polymerization catalytic component comprising a half metallocene complex having a Group 3 metal or a lanthanoid metal (hereinafter, a Group 3 metal and a lanthanoid metal are collectively referred to as rare earth metals) as a central metal can be used in the polymerization or copolymerization of ethylene and α-olefins, the polymerization or copolymerization of aromatic olefin monomers such as styrene, and the polymerization or copolymerization of cyclic olefin monomers such as norbornene (see WO 2006/004068). It is also known that such a half metallocene complex having a rare earth metal as a central metal can be used in the polymerization of cyclohexene oxide, and the copolymerization of cyclohexene oxide and carbon dioxide (see Macromolecules 2005, 38, 4089). It is known that a half metallocene complex having lanthanum as a central metal can be used in the syndiotactic polymerization of a methacrylate ester, the block copolymerization of ethylene and a methacrylate ester, the polymerization of isocyanide, and the polymerization of acrylonitrile (see J. Polym. Sci.: Part A: Polym. Chem. 2001, 39, 1382). Thus, the half metallocene complex having a rare earth metal as a central metal is a useful metal complex for which many utilities are expected. On the other hand, as a metal complex having a ligand in which a cyclopentadiene derivative and a phenol derivative are linked with a Group 14 atom, a Group 4 transition metal complex is known as a catalyst for olefin polymerization (see JP-A-09-87313), but no rare earth metal complex is known.
The polymers of lactone compounds such as ε-caprolactone which can be industrially produced at low cost are useful as modifiers for polyurethane, coating resins and plastics. Among those polymers, particularly the lactone polymer with a high molecular weight attracts attention as a biodegradable polymer. As a process for producing the high-molecular weight polymer of a lactone, a method of ring-opening polymerization of a lactone using a rare earth alkoxide as a catalyst (see WO 91/05001), or a method of ring-opening polymerization of a lactone using a rare earth complex having a cyclopentadiene ligand as a catalyst is known (see JP-A-05-247184). After that, regarding the ring-opening polymerization of a lactone using a rare earth complex as a catalyst, efforts for efficiently obtaining a polymer having a narrow molecular weight distribution have been reported (see Macromolecules, 1996, 29, 1798).